p38 MAPK is activated but not necessary in porcine von Willebrand factor-dependent platelet activation

We have investigated the role of p38 mitogen-activated protein kinase (MAPK ) in von Willebrand factor (VWF)-dependent platelet activation. The interac tion of platelets with subendothelial VWF. especially under high shear stre ss. is considered to be the first activation step which primes platelets fo r subsequent haemostatic events. As a model of VMF-dependent platelet activ ation, porcine VWF was employed. Porcine VWF induced p38 MAPK activation by 1 min post-addition; assessed by phosphorylation of a recombinant p38 MAPK fusion protein substrate termed glutathione S-transferase-MAPK activated p rotein kinase-2. To determine if p38 MAPK was necessary for porcine VWF-ind uced platelet activation. we functionally inhibited p38 MAPK activity with SB203580 before exposure of the platelets to porcine VWF Inhibition of p38 MAPK had no effect on VWF-induced platelet alpha or lysozomal granule relea se, expression of activated GPIIb IIIa, modulation of membrane glycoprotein CD41, expression of phosphatidyl-serine as assessed by annexin V binding, microparticle formation, or platelet agglutination. It was concluded that S B203580-inhibitable p38 MAPK activity induced by porcine VWF is not necessa ry for platelet activation.